Faced with numerous complaints across the country over foul smell from bio-toilets in train compartments spreading to passenger seats, engineers at the Southern Railway’s carriage workshop at Perambur have tweaked the design of the exit pathway to resolve the issue.
The modification, known as S Trap, has been made by employing a simple, yet effective concept in fluid mechanics known as ‘siphon effect’. This utilises the momentum generated by the flow of the water to push the faecal matter into the bio-toilet tank. The arrangement also maintains an effective ‘water seal’ between the tank and toilet, which prevents emanation of foul smell.
Passengers seated on berths nearest to the toilets were the worst affected by the foul smell emanating from the bio-toilets, with the Railways receiving a multitude of complaints regarding this, officials stated, explaining the reason for the alternative design.
In the earlier design, the faecal matter would drop down towards a a ball valve and racket pinion arrangement, also known as a P trap (see illustration). The night soil would have to be travel diagonally upwards by the force of the water column above it. The water column was supposed to maintain the water seal to block the foul smell from the tank into the lavatory. But, the faecal matter deposited on the ball-valve led to its corrosion, which breaks the water seal, leading to foul smell from the tank spreading into the coach. The S trap design has completely done away with this ball valve arrangement and used a horizontal S design for the chamber (see illustration), which is also narrower.
This also eliminates another major issue faced by railway officials with the earlier design of bio-toilets where napkins, beer bottles and other such waste dumped by passengers into the toilet opening led to choking of ball valve arrangement, railway officials said.
The S trap arrangement has been in-principle approved by the Railway Board Member (Mechanical) on his recent visit to Chennai and has been installed in three coaches in Southern Railway. The performance is being monitored on a day-to-day basis and is likely to be implemented in trains running across the country after a 6- month trial run, railway officials said.